In the technical field of photon detection, there is a need for devices that will enable high-sensitivity detection of electromagnetic radiation, and hence enable detection of an even limited number of photons associated with the electromagnetic radiation itself.
For the above purpose, the so-called Geiger-mode avalanche photodiodes (GM-APD) are known, which theoretically enable detection of single photons.
A Geiger-mode avalanche photodiode, also known as single-photon avalanche diode (SPAD), is formed by an avalanche photodiode (APD), and hence comprises a junction, typically of the P-I-N type, and an additional region of semiconductor material slightly doped, alternatively, P or N; the additional region is set between the intrinsic region I and the region of the P-I-N junction having a conductivity of an opposite type with respect to the conductivity of the additional region itself so as to provide a structure formed by regions made of semiconductor material that present a succession according to a P-N-I-N or else N-P-I-P scheme.
On the basis of the physical phenomena that arise, the additional region is also known as multiplication region in so far as it is the site of phenomena of impact ionization, whilst the intrinsic region is also known as absorption region, since the majority of the photons are absorbed therein.
In greater detail, the junction has a breakdown voltage VB and is biased, in use, with a reverse-biasing voltage VA higher in magnitude than the breakdown voltage VB of the junction, typically higher by 10-20%. In this way, the generation of a single electron-hole pair, following upon absorption of a photon impinging on the SPAD, is sufficient for triggering an ionization process that causes an avalanche multiplication of the carriers, with gains of around 106 and consequent generation in short times (hundreds of picoseconds) of the avalanche current. This avalanche current may be appropriately collected, typically by means of a external circuitry coupled to the junction, for example, by means of anode and cathode contacts, and represents an output signal of the SPAD.
To be precise, across the junction an effective voltage Ve is present, which may coincide with the reverse-biasing voltage VA only in the absence of photons. In fact, in the presence of photons, and hence of current generated inside the SPAD, the effective voltage Ve across the junction may be lower, in magnitude, than the reverse-biasing voltage VA. However, in the present document it is assumed, except where otherwise expressed explicitly, that the effective voltage Ve across the junction coincides or approximately coincides with the reverse-biasing voltage VA.
The gain and likelihood of detection of a photon, i.e., the sensitivity of the SPAD, are directly proportional to the value of reverse-biasing voltage VA applied to the SPAD. In fact, the more the reverse-biasing voltage VA exceeds, in magnitude, the breakdown voltage VB, the higher the likelihood of an avalanche generation of charge carriers to occur.
However, high reverse-biasing voltages VA allow, even in the absence of incident photons (dark conditions), a single charge carrier, generated for example by transfer of thermal energy, to be sufficient to trigger the avalanche-ionization process, generating so-called dark current, which may adversely interfere with the normal use of the SPAD.
In addition, the fact that the reverse-biasing voltage VA is appreciably higher than the breakdown voltage VB may cause the avalanche-ionization process, once triggered, to be self-sustaining. Consequently, once triggered, the SPAD is no longer able to detect photons, with the consequence that, in the absence of appropriate remedies, the SPADs described may manage to detect arrival of a first photon, but not arrival of subsequent photons.
To be able to detect also these subsequent photons, one may quench the avalanche current generated inside the SPAD, stopping the avalanche-ionization process. In detail, one may lower, for a period of time known as a “hold-off time”, the effective voltage Ve across the junction, so as to inhibit the ionization process and quench the avalanche current, as described hereinafter. Next, the initial conditions of biasing of the junction are restored so that the SPAD will be again able to detect photons. Since during the hold-off time the SPAD is not able to detect photons, it is desirable for it to be as short as possible.
To lower the effective voltage Ve across the junction following upon absorption of a photon, SPADs may have a so-called quenching circuit.
Amongst other things, there are known quenching circuits of a passive type, comprising a quenching resistor set in series with the junction and having a resistance on the order of hundreds of kilo-ohms.
In the absence of photons, the presence of the quenching resistor does not alter the effective voltage Ve across the junction, which may be equal to the reverse-biasing voltage VA. However, following absorption of a photon and a consequent triggering of the ionization process, the avalanche current that originates therefrom causes, by flowing in the quenching resistor, a reduction of an exponential type of the effective voltage Ve across the junction, which voltage decreases until it reaches a value that may be just a little higher than the breakdown voltage VB. As regards, instead, the avalanche current, immediately after triggering of the avalanche-ionization process (turning-on of the SPAD), it passes from a zero value to a peak value, then decreases exponentially towards an asymptotic value, which is inversely proportional to the resistance of the quenching resistor and directly proportional to the difference between the reverse-biasing voltage VA and the breakdown voltage VB, this difference being generally known as overvoltage (OV).
In detail, it may be shown that the avalanche-ionization process is arrested in the case where the value of the avalanche current drops below a threshold value known as latching current I. Consequently, given a quenching resistor having a resistance Rq, it is able to quench the avalanche current in the event of overvoltage equal at the most to the product Rq·I, i.e., in the event of reverse biasing voltages VA not higher than VB+Rq·I. If these conditions are respected, the avalanche current is typically quenched; next, the SPAD, the behavior of which is to a certain extent comparable to that of a capacitor, recharges exponentially through the quenching resistor, in such a way that the effective voltage Ve across the junction returns to being equal to the reverse-biasing voltage VA, the SPAD thus being ready to detect the arrival of a new photon. During the recharging time, i.e., in the time interval in which the effective voltage Ve increases exponentially until it returns to being equal or approximately equal to the reverse-biasing voltage VA, the SPAD has a reduced sensitivity, which increases as the effective voltage Ve increases.
On the basis of what has been said, it may be inferred that applications that are particularly demanding in terms of sensitivity typically require high overvoltages OV, hence high reverse-biasing voltages VA, with the consequence that the quenching resistor must assume high values; otherwise, it would be difficult to impossible to quench the avalanche current and thus detect subsequent photons.
Quenching resistors with high resistances entail a recharge time that is longer than what may be obtained in the presence of lower resistances; however, they entail an additional advantage. In fact, during the recharge time the SPAD is certainly less sensitive to arrival of the photons, but is also less sensitive to the afterpulsing phenomenon, which usually degrades the performance of the SPADs. In detail, the afterpulsing phenomenon consists of the secondary emission of carriers owing to the presence of lattice defects in the depletion region, which create intermediate energy levels (comprised between the conduction band and the valence band) that can capture one or more carriers of the avalanche current, then releasing them with unpredictable delays, causing an increase in the dark current and distorting the output signal of the SPAD.
Typically, the value of resistance of the quenching resistor is sized as a function of the required recharging times and sensitivity to the afterpulsing phenomenon, as well as the type of application envisaged for the SPAD.
Similar considerations may be made as regards the so-called SPAD arrays, and moreover as regards the so-called silicon photomultipliers (SiPMs), used in order to improve the performance that can be obtained with individual SPADs.
In particular, an SPAD photodiode array, two examples of which are shown in the U.S. Publication No. 2009/0184384 and U.S. Publication No. 2009/0184317 and which are incorporated by reference, is formed by a planar array of SPADs grown on one and the same substrate.
An SiPM is a particular SPAD array. In detail, the SiPM is formed by an SPAD array grown on one and the same substrate and provided with respective quenching resistors (for example, of a vertical type) integrated in the SPADs, these quenching resistors being uncoupled from and independent of one another. In addition, the anode and cathode contacts of each SPAD are configured so that they can be coupled to a single voltage generator. Consequently, the SPADs of the SiPM can be biased at one and the same reverse-biasing voltage VA. In addition, the avalanche currents generated therein are multiplied together so as to generate an output signal of the SiPM equal to the summation of the output signals of the SPADs. As regards the terminology, in the technical field of SiPMs it is common to refer to the ensemble formed by the photodiode and the quenching resistor as pixel, the SiPM being hence formed by an array of pixels.
The SiPM is hence a device with a large area and high gain, capable of supplying, on average, an electrical output signal (current) proportional to the number of photons that impinge on the SiPM. In fact, since the quenching resistors are uncoupled from one another, each photodiode of the SiPM behaves as an independent binary counter, whilst the output signal of the SiPM is proportional to the number of pixels activated, i.e., to the number of SPADs where the avalanche-ionization process (detection of a photon) is triggered, this number being in turn proportional to the number of incident photons.
In order to obtain an SPAD and an SiPM with flexibility of use, U.S. patent application Ser. No. 12/637,628, which is incorporated by reference, describes an SPAD, which has a body made of semiconductor material with a first type of conductivity, and forms a first surface and a second surface. In addition, the SPAD has a trench that extends through the body starting from the first surface and surrounds an active region. Present within the trench is a lateral insulation region, which comprises a conductive region of metal material and an insulation region, the latter being made of dielectric material and surrounding the conductive region. Extending within the active region, starting from the first surface, is an anode region of a second type of conductivity. In addition, the active region forms a cathode region, which extends between the anode region and the second surface and defines a vertical quenching resistor. The SPAD further comprises a contact region made of conductive material overlying the first surface and in direct contact with the conductive region present inside the trench.
By electrically connecting the contact region to an external biasing circuit, it is possible to bias the contact region itself to a gate voltage VG. In this way, a depletion region is created around the insulation region and internally around the active region. Consequently, by varying the gate voltage VG, it is possible to modulate the extent of the depletion region, consequently varying the resistance of the quenching resistor. In this way, it is possible to adapt each time the SPAD, and in particular the resistance of the quenching resistor, to the requirements of the application envisaged for the SPAD.
Even though the SPAD described represents a considerable improvement in terms of adaptability of the photodiode to the type of application, it envisages the use of at least one epitaxial layer with a low level of doping (in the limit, intrinsic) and large thickness (of the order of 50-60 μm), in order to provide values of resistance of the quenching resistor that are sufficiently high. Consequently, the trench may have a large depth, and hence a high aspect ratio, i.e., a high depth/width ratio; in fact, the width of the trench may be on the order of microns and typically cannot be increased; otherwise, the overall geometrical dimensions may increase. Also, the provision of trenches with high aspect ratios may require non-standard technological processes, such as, for example, repetition of cycles of passivation and etching carried out in an environment rich in fluorine, as well as the use of non-conventional machinery, with a consequently higher complexity of production. In addition, filling of these trenches with metal material in order to provide the conductive region of metal material may be technologically complex.